1. Field of the Invention
The present invention relates to a light guide plate for a surface light source, which is used as a backlight or the like of a liquid crystal display.
2. Description of the Related Art
As a light guide plate for a surface light source used as a backlight of a liquid crystal display, for example, there is already known one disclosed in Japanese Patent Application Laid-Open No. 2001-35224 (hereinafter, referred to as “Patent Document 1”) or Japanese Patent Application Laid-Open No. 2006-49286 (hereinafter, referred to as “Patent Document 2”). The light guide plate in Patent Document 1 has an incident surface of light source light, which is one side surface of the light guide plate, and one of both surfaces (the front surface and rear surface) in the thickness direction of the light guide plate is a reflective surface (a surface on which a reflective sheet is polymerized) for reflecting light guided into the light guide plate and the other is a light emitting surface. In this instance, dots which are a plurality of fine convex or concave portions are formed on the reflective surface or the emitting surface. The dots are used to scatter the light guided into the light guide plate to adjust the light traveling direction in order to emit the light guided into the light guide plate from the emitting surface. In this instance, the number of dots per unit area is increased as the distance from the light incident surface increases in order to uniform the luminance of the light emitted from the emitting surface of the light guide plate. In some cases, however, the area (the area viewed in the thickness direction of the light guide plate) of each dot may be increased along with an increase in the distance from the light incident surface.
In the light guide plate disclosed in Patent Document 2, a plurality of prisms (roulette) extending in a direction parallel to the light incident surface are formed on one of both surfaces of the light guide plate in the thickness direction thereof in parallel with the direction perpendicular to the light incident surface of the light guide plate. Moreover, the reflection or refraction of light on an inclined surface of the prism is used to adjust the traveling direction of the light guided into the light guide plate before the emission of the light from a light emitting surface of both surfaces in the thickness direction of the light guide plate. In this instance, the luminance of the light emitted from the light emitting surface is uniformed by decreasing the pitch of the prisms in the direction perpendicular to the light incident surface along with an increase in the distance from the light incident surface or by increasing the width of the inclined surface on the light incident surface side of the prism viewed in the thickness direction of the light guide plate along with an increase in the distance from the light incident surface.
In the dots in Patent Document 1 and the light guide plate with the prisms formed on the front or rear surface as disclosed in Patent Document 2, however, there has been the following problem.
Specifically, the light guide plate for a surface light source used as a backlight of a liquid crystal display is required to have appropriate light distribution characteristics similar to target light distribution characteristics as far as possible, with respect to the light distribution characteristics of the emitting direction or the degree of directivity of the light from the light emitting surface of the light guide plate.
In the light guide plate having dots on the front or rear surface as in Patent Document 1 (hereinafter, the front or rear surface of the light guide plate having the dots is sometimes referred to as “dot formation surface”), the light distribution characteristics of the emitted light vary according to the angle of inclination of a side wall surface of each dot in the above situation. Therefore, to achieve the target light distribution characteristics, it is necessary to set the angle of inclination of the side wall surface of each dot to a desired angle in advance. For example, to adjust the light distribution characteristics of the emitted light viewed in the longitudinal direction of the light incident surface to the target light distribution characteristics, it is necessary to set the angle of inclination of the side wall surface on the light incident surface side of each dot and the angle of inclination of the side wall surface on the side opposite to the light incident surface side to appropriate angles of inclination, respectively, in advance (the angle of inclination differs for each side wall surface in general).
Moreover, in the light guide plate having prisms on the front or rear surface as in Patent Document 2 (hereinafter, the front or rear surface of the light guide plate having the prisms is sometimes referred to as “prism formation surface”), the light distribution characteristics of the emitted light (the light distribution characteristics viewed in the longitudinal direction of the light incident surface) vary according to the angles of inclination of the inclined surfaces (an inclined surface on the light incident surface side and an inclined surface on the side opposite thereto) of each prism. Therefore, to achieve the target light distribution characteristics, it is necessary to set the angle of inclination of the inclined surface on the light incident surface side of each prism and the angle of inclination of the inclined surface on the side opposite to the foregoing inclined surface to appropriate angles of inclination (the angle of inclination differs for each inclined surface in general), respectively.
The light guide plate having the dot formation surface and the prism formation surface is generally formed by injection molding or the like by using a stamper (molding die) having a mold surface formed into a shape corresponding to the dot formation surface or the prism formation surface.
In this situation, it is very difficult to form a plurality of concave or convex portions corresponding to the dots on the formation surface by cutting or the like by using the stamper for use in forming the dot formation surface on the light guide plate. Therefore, the concave or convex portions are formed generally by exposure using photoresist and masking. Accordingly it is difficult to form a plurality of dots each having a desired shape on the dot formation surface of the light guide plate with accuracy. Particularly, it is extremely difficult to adjust the angle of inclination of the side wall surface of each dot to an appropriate angle of inclination for each portion of the side wall surface with accuracy.
Consequently, in the conventional light guide plate having the dot formation surface, the angle of inclination, but exclusive of the angle of inclination of a part of the side wall surfaces of each dot (for example, the side wall surface on the light incident surface side), often becomes an angle of inclination inappropriate for adjusting the light distribution characteristics of the light emitted from the light guide plate to the target light distribution characteristics. In addition, an edge of the side wall surface of a dot will be rounded in general, which easily causes light scattering in the portion. Moreover, an increase in dot density (the number of dots per unit area) on the dot formation surface of the light guide plate reduces the side wall surfaces in the portion where an overlap occurs between dots adjacent to each other. Therefore, it is impossible to largely increase the dot density and thus impossible to sufficiently increase the proportion of the dot side wall surfaces useful for controlling the light distribution of the emitted light on the dot formation surface of the light guide plate.
Given this situation, the conventional light guide plate with the dot formation surface easily emits a large amount of light in a direction largely deviating from the light emitting direction of the target light distribution characteristics (the light is sometimes referred to as “lost light”). Thereby, it has been difficult to emit the light guided into the light guide plate efficiently from the light guide plate with the target light distribution characteristics. Moreover, it is difficult to adjust the angle of inclination of the side wall surface of each dot to a desired angle of inclination for each portion of the side wall surface, which thus leads to a difficulty in adjusting both of the light distribution characteristics of the emitted light viewed in the longitudinal direction of the light incident surface and the light distribution characteristics of the emitted light viewed in the direction perpendicular to the light incident surface to the target light distribution characteristics.
On the other hand, in the light guide plate having the prism formation surface, it is possible to form a mold surface having a shape for a desired-shaped prism with relatively accuracy on the stamper for forming the prism formation surface by cutting or the like. Therefore, it is possible to adjust the angle of inclination of the inclined surface of each prism on the prism formation surface of the light guide plate to an appropriate angle of inclination with accuracy.
In the conventional light guide plate having the prism formation surface, however, the prisms merely extend in the longitudinal direction of the light incident surface. This causes a problem that it is impossible to adjust the light distribution characteristics of the emitted light viewed in the direction perpendicular to the light incident surface or the luminance distribution of the emitted light in the longitudinal direction of the light incident surface to the target light distribution characteristics or luminance distribution.